The invention discloses a method for reducing a minimum line width in a spacer-defined double patterning process of the present invention. In the method, the silicon nitride spacers can be converted into trenches in the interlayer dielectric layer by using a silicon dioxide film as a mask and by means of a chemically mechanical polishing process and an etching process, so that the minimum line width of the trenches can be determined by the width of the silicon nitride spacers, and thus a smaller line width can be achieved and the process can be simple and easy to control.